Membrane-enclosed biological structures can support a voltage difference between the inside and the outside of the membrane. This voltage, also called membrane potential, serves a variety of biological functions, including carrying information (e.g., in neurons), acting as an intermediate in production of ATP (e.g., in bacteria and mitochondria), powering the flagellar motor (e.g., in bacteria), and controlling transport of nutrients, toxins, and signaling molecules across the cell membrane (in bacteria and eukaryotic cells).
In spite of its fundamental biological role, membrane potential is very difficult to measure. Electrophysiology involves positioning electrodes on both sides of the membrane to record voltage directly. Electrophysiological experiments are slow to set up, can only be performed on one or a few cells at a time, cannot access deeply buried tissues (e.g., in vivo), do not work for cells that are too small (e.g. bacteria) or are enclosed in a hard cell wall (e.g. yeast), or are motile (e.g., sperm) cannot be applied to long-term measurements, and usually damage or kill the cell under study.
Accordingly, novel methods for measuring membrane potential are needed.